From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-3.8 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI, SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id A2E69C4363A for ; Thu, 29 Oct 2020 13:31:02 +0000 (UTC) Received: from merlin.infradead.org (merlin.infradead.org [205.233.59.134]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPS id 02650207DE for ; Thu, 29 Oct 2020 13:31:01 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=lists.infradead.org header.i=@lists.infradead.org header.b="gwY/M6R2" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 02650207DE Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=arm.com Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=lists.infradead.org; s=merlin.20170209; h=Sender:Content-Transfer-Encoding: Content-Type:Cc:List-Subscribe:List-Help:List-Post:List-Archive: List-Unsubscribe:List-Id:In-Reply-To:MIME-Version:References:Message-ID: Subject:To:From:Date:Reply-To:Content-ID:Content-Description:Resent-Date: Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID:List-Owner; bh=RPEL4XGW+6Qt4Zwe0C2kYWOjlLGyB4xe45skcOgPSfM=; b=gwY/M6R2OvMOEu+wQvvedbwZu +SHANAbM6Z8wOS7lveV3EoZWGNco7fpDaZZvgwJ+oxspkwWyt2tB1zDHvNUdV/0lD4UvI+OS8h7mS QEFyi3Uuo53tDUhFnfEL6KFqv63XL55mw1zBTml2k6djBTKB/RSwEzFRIWJvTMDeja4LwtzA4vuGt 73asSYd8dz1BFpWyN3kOpxZipFLiM9eB3FnSXPtWNydKILEfAbWoYV+qu6PS/ocUJIK9jbQqQ9f13 napQ9pOz55Rwj605LFYg4fzUiflq9DVaY55u3oDwKq3lR31gxkGEK4pN2GTnopSFz5J9xYUfNZew4 Pz5h3s1Tg==; Received: from localhost ([::1] helo=merlin.infradead.org) by merlin.infradead.org with esmtp (Exim 4.92.3 #3 (Red Hat Linux)) id 1kY80U-0005XZ-HM; Thu, 29 Oct 2020 13:30:22 +0000 Received: from foss.arm.com ([217.140.110.172]) by merlin.infradead.org with esmtp (Exim 4.92.3 #3 (Red Hat Linux)) id 1kY80R-0005Wo-0M for linux-arm-kernel@lists.infradead.org; Thu, 29 Oct 2020 13:30:20 +0000 Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id 218DE139F; Thu, 29 Oct 2020 06:30:17 -0700 (PDT) Received: from C02TD0UTHF1T.local (unknown [10.57.55.248]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 753A23F719; Thu, 29 Oct 2020 06:30:15 -0700 (PDT) Date: Thu, 29 Oct 2020 13:30:12 +0000 From: Mark Rutland To: Peter Zijlstra Subject: Re: [PATCH v2] arm64: implement support for static call trampolines Message-ID: <20201029133012.GC61831@C02TD0UTHF1T.local> References: <20201028184114.6834-1-ardb@kernel.org> <20201029115026.GA61831@C02TD0UTHF1T.local> <20201029115832.GO2628@hirez.programming.kicks-ass.net> MIME-Version: 1.0 Content-Disposition: inline In-Reply-To: <20201029115832.GO2628@hirez.programming.kicks-ass.net> X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20201029_093019_165479_91CE72EF X-CRM114-Status: GOOD ( 19.07 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: catalin.marinas@arm.com, will@kernel.org, Ard Biesheuvel , linux-arm-kernel@lists.infradead.org, james.morse@arm.com Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org On Thu, Oct 29, 2020 at 12:58:32PM +0100, Peter Zijlstra wrote: > On Thu, Oct 29, 2020 at 11:50:26AM +0000, Mark Rutland wrote: > > Hi Ard, > > > > On Wed, Oct 28, 2020 at 07:41:14PM +0100, Ard Biesheuvel wrote: > > > Implement arm64 support for the 'unoptimized' static call variety, > > > which routes all calls through a single trampoline that is patched > > > to perform a tail call to the selected function. > > > > Given the complexity and subtlety here, do we actually need this? > > Only if you can get a performance win. The obvious benefit is loosing > the load that's inherent in indirect function calls. The down-side of > the indirect static-call implementation is that it will incur an extra > I$ miss. > > So it might be a wash, loose a data load miss, gain an I$ miss. I reckon it'll be highly dependent on microarchitecture since it'll also depend on how indirect branches are handled (with prediction, forwarding, speculation, etc). I don't think we can easily reason about this in general. > The direct method (patching the call-site, where possible) would > alleviate that (mostly) and be more of a win. I think that where the original callsite can be patched with a direct branch, it's desireable that we do so. That's simple enough, and there are places where that'd be useful from a functional pov (e.g. if we want to patch branches in hyp text to other hyp text). However, if the range of the branch requires a trampoline I'd rather the trampoline (and the procedure for updating it) be as simple as possible. Thanks, Mark. _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel